An aqueous resin is used in various fields including paints, inks, fiber treating agents, adhesives, coating agents and the like, but is inferior in water resistance to an oily resin as it is aqueous. Therefore, in order to enhance the water resistance of the aqueous resin, a method using an aqueous resin crosslinking agent has been conventionally employed, and compounds having a carbodiimide group, a methylol group, an ethyleneimine group, an epoxy group, an isocyanate group or the like are used as the aqueous resin crosslinking agent.
Among them, the carbodiimide-based aqueous resin crosslinking agent having a carbodiimide group is advantageous in that it can react with the functional group (for example, a carboxyl group) contained in the aqueous resin at a low temperature to enhance the properties such as strength, water resistance and adhesion of the coating film formed from the aqueous resin.
However, when such a carbodiimide-based aqueous resin crosslinking agent is added to the aqueous resin, the carbodiimide group in the crosslinking agent reacts with the carboxyl group in the aqueous resin and an aqueous medium such as water with time, leading to gradual gelation and insufficient crosslinking, and it is thus necessary to perform a crosslinking reaction, for example by heating, immediately after the addition of the crosslinking agent.
In recent years, it is desired to store, transport and market the mixture of a carbodiimide-based aqueous resin crosslinking agent and an aqueous resin. Therefore, there is a need for carbodiimide-based aqueous resin crosslinking agents excellent in the storage stability that can perform the crosslinking reaction sufficiently even after mixture of the carbodiimide-based aqueous resin crosslinking agents and the aqueous resin and storage for a long period of time.
As such a carbodiimide-based aqueous resin crosslinking agent, PTL1 describes aqueous dicyclohexylmethanecarbodiimide having a specific structure. PTL2 describes a method of producing a carbodiimide-based aqueous resin crosslinking agent comprising dispersing a mixture of a particular compound having a carbodiimide group and a particular radically polymerizable unsaturated monomer in water with a surfactant to give a dispersed particle and then polymerizing the radically polymerizable unsaturated monomer.
PTL3 describes, as a carbodiimide-based aqueous resin crosslinking agent, a hydrophilic polycarbodiimide compound having a lipophilic molecular chain attached at one terminal of a molecular chain having a carbodiimide group and having a hydrophilic molecular chain attached at the other terminal thereof. PTL4 describes a water-soluble or water-dispersible modified polycarbodiimide amine obtained by modifying, a polycarbodiimide derived from an aromatic diisocyanate compound having its terminal blocked with a hydrophilic compound, with a secondary amine.
PTL5 discloses a method of producing a microcapsule having a capsule wall and a capsule core, and also discloses a resin crosslinking agent the storage stability of which is enhanced by using a crosslinking agent as the capsule core.